A study is being conducted to compare cases of acute myocardial infarction served in the pre-hospital period by Mobile Coronary Care Units (MCCU), with acute myocardial infarction cases arriving at the same hospitals by other means. The data being collected include an extensive interview with the patient and/or the patient's family and associated to ascertain prior health status of the patient; the prodromal period; the acute episode; and the actions taken from the time of initial awareness of the problem to entry into the hospital. For those served by the Mobile Coronary Care Unit, recordings are also obtained of the cells received at the Emergency Communication Center that led to the dispatch of the MCCU. For both subsamples further information is obtained from the emergency room and hospital records. Abstracts of the hospital record, including electrocardiograms and laboratory findings, are reviewed by cardiologists, who classify cases in terms of cardiac diagnosis. Patients discharged alive from the hospital are followed and their status determined six months after the initial episode. Reports of the emergency squadmen are reviewed with respect to actions they carried out in the pre-hospital phase. It is expected that the total sample will include five hundred MCCU cases and an equal number of control cases.